This article discusses the role of CBP/p300 in cell growth, transformation, and development. Initially identified through their interactions with transcription factors and viral proteins, CBP and p300 are highly conserved and involved in various cellular functions. The article highlights their dual roles as both tumor suppressors and oncogenic coactivators, depending on the cellular context. Evidence suggests that CBP and p300 participate in tumor-suppressor pathways, as seen in mice with CBP mutations that develop hematological malignancies. However, they also interact with oncogenes and are essential for the actions of many oncogenes. CBP/p300 are involved in p53-mediated functions, including transcriptional activation and regulation of p53 degradation. The article also explores their role in chromosomal translocations associated with leukemia, such as the t(8;16) and MLL-CBP fusions, which disrupt normal cellular functions and lead to transformation. Additionally, the article discusses the involvement of CBP/p300 in the actions of viral oncoproteins like E1A, SV40 T antigen, and HPV E6, which either require CBP/p300 for their functions or inhibit them. The regulation of CBP/p300 by phosphorylation and their involvement in developmental processes, particularly in model systems like *Drosophila*, are also addressed. Overall, the article emphasizes the complex and context-dependent roles of CBP/p300 in cellular processes, highlighting their significance in both normal development and disease.This article discusses the role of CBP/p300 in cell growth, transformation, and development. Initially identified through their interactions with transcription factors and viral proteins, CBP and p300 are highly conserved and involved in various cellular functions. The article highlights their dual roles as both tumor suppressors and oncogenic coactivators, depending on the cellular context. Evidence suggests that CBP and p300 participate in tumor-suppressor pathways, as seen in mice with CBP mutations that develop hematological malignancies. However, they also interact with oncogenes and are essential for the actions of many oncogenes. CBP/p300 are involved in p53-mediated functions, including transcriptional activation and regulation of p53 degradation. The article also explores their role in chromosomal translocations associated with leukemia, such as the t(8;16) and MLL-CBP fusions, which disrupt normal cellular functions and lead to transformation. Additionally, the article discusses the involvement of CBP/p300 in the actions of viral oncoproteins like E1A, SV40 T antigen, and HPV E6, which either require CBP/p300 for their functions or inhibit them. The regulation of CBP/p300 by phosphorylation and their involvement in developmental processes, particularly in model systems like *Drosophila*, are also addressed. Overall, the article emphasizes the complex and context-dependent roles of CBP/p300 in cellular processes, highlighting their significance in both normal development and disease.